The present invention relates to a method of bonding flying leads, more precisely relates to a method of bonding flying leads to pads of a board by using supersonic vibrations.
A carriage assembly of a magnetic disk drive unit is shown in FIG. 8. The carriage assembly includes a plurality of carriage arms 10, whose number corresponds to that of magnetic disks, and suspensions 12, on which magnetic heads are mounted, are attached to front ends of the carriage arms 10. Base ends of the carriage arms 10 are attached to an actuator shaft 14, and the carriage arms 10 are turned about the shaft 14 and moved parallel to surfaces of the magnetic disks.
The magnetic heads mounted on the suspensions 12 are electrically connected to a signal transmission circuit by several manners. FIG. 8 shows a connecting structure using a so-called long tail suspension board, wherein an end of a suspension board of the suspension 12 is extended to a location of attaching a flexible board 16, which is attached to side faces of the base ends of the carriage arms 10, and ends of the suspension board is formed into flying leads.
In the connecting structure using the long tail suspension board, pads of the flexible board 16 and the flying leads 18 (see FIG. 9) of the long suspension board are correctly positioned, then the flying leads 18 are bonded to the pads by a supersonic bonding tool. In FIG. 9, the flying leads 18 are supersonic-bonded to the pads 17 of the flexible board 16 by the bonding tool 20.
The supersonic bonding method has been used for bonding a semiconductor chip to a circuit board by flip-chip connection, bonding wires to leads, etc. To securely perform the supersonic bonding, several ideas have been proposed. For example, Japanese Patent Gazette No. 10-150137 discloses a method of bonding wires, wherein a leadframe is pressed by a vibration restraining member so as to prevent resonance of the leadframe; Japanese Patent Gazette No. 2005-136399 discloses a method of forming bonding-electrodes, wherein an electrically conductive material is applied to electrodes of a circuit board so as to broaden a bonding area; Japanese Patent Gazettes No. 08-146451 and No. 10-189657 disclose methods of bonding two members, wherein an anisotropic conductive film is provided between the members, and supersonic waves are applied in the direction for mutual contact; Japanese Patent Gazette No. 05-63038 discloses a method of bonding two members, wherein their bonding faces are made rough; and Japanese Patent Gazette No. 2005-93581 discloses a method of bonding two members, wherein non-conductive adhesive is applied to bonding faces.
In the connecting structure using the long tail suspension board shown in FIG. 8, a plurality of the flying leads 18 are arranged parallel with minute separations. The flying leads 18 may be supersonic-bonded, one by one, to the pads 17, but it is efficient to simultaneously supersonic-bond a plurality of the flying leads 18 as shown in FIG. 9.
In FIG. 9, the bonding tool 20 contacts and bonds two flying leads 18. However, in case that the bonding tool 20 contacts a plurality of the flying leads 18, a working face of the bonding tool 20 is a flat face. So, if asperities exist in the bonding face, bonding strength differs at every bonding point, so that bonding reliability must be lowered.
FIG. 10 is a sectional view of the flying lead 18 and the pad 17, which have been mutually bonded. The boundary includes three kinds of parts: (1) completely bonded parts, wherein projections are crushed and oxide film is broken, so that the bonding faces are actually bonded each other; (2) poorly bonded parts, wherein the oxide film exists between the bonding faces; and (3) nonbonded parts “A”, wherein the bonding faces are not bonded each other.
Outer surfaces of the flying leads 18 and the pads 17 are gold-plated, so that they are connected by gold-gold bonding. The gold plated layers are capable of absorbing the asperities formed in the bonding faces of the flying leads 18 and the pads 17. However, thickness of the gold layers are about 3 μm, so all of the asperities cannot be fully absorbed.